1. Field of the Invention
The present invention relates to a catalyst for the high efficiency purification of exhaust gases discharged from an auto internal combustion engine by the removal of such harmful constituents as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx) from the gases. The invention also relates to a method of preparing the catalyst.
2. Description of the Prior Art
Catalysts which are designed for use in the purification of exhaust gases discharged from an auto internal combustion engine or the like should be highly active over a wide range of temperatures, because the catalysts are used under service conditions in which the chemical reaction-rate determining factors such as the volume and concentration of reactants or the working temperature cannot be maintained constant.
Catalyst formulations in which a catalyst carrier is coated with an active substance and further wherein a catalytic material is supported on said carrier are well known as catalysts suitable for the purification of exhaust gas. Active alumina, principally .gamma.-alumina, has been known to be a particularly effective material as an active thin film having a large surface area for the coating of an underlying catalyst carrier. However, .gamma.-alumina is very likely to suffer changes in crystal structure over long service lives at high temperatures, which results in a decrease of the specific surface area of the catalyst, substantial loss in catalytic activity of the catalyst over periods of time and losses in durability. Moreover, .gamma.-alumina reacts with such catalyst poisons as compounds of lead, sulfur and phosphorus which are present in the discharged gas. Thus, .gamma.-alumina is not always satisfactory in terms of anti-toxicity characteristics. Moreover, for catalysts of a catalytic oxide of a metal such as nickel, iron or mixtures thereof supported on .gamma.-alumina, long service lives at high temperature cause the metal oxides to react with .gamma.-alumina, which results in deterioration of catalyst activity. A need, therefore, continues to exist for an exhaust gas purification catalyst which exhibits stable and efficient activity over long service life.